Magnetic field measuring apparatus

ABSTRACT

A beam that passes through a plurality of gas cells a number of times is led to a deflection meter from a light ejecting section, detection of a deflected surface angle is performed and a strength of a magnetic field is measured by a structure in which the plurality of the gas cells is arranged along a light beam between two reflection units or light concentrating units that have a light beam incidence section and a light beam ejecting section and are opposite to each other, and a laser beam that is incident from the light beam incidence section passes through the plurality of the gas cells and then is multiply reflected by both reflection units.

CROSS-REFERENCE TO RELATED APPLICATIONS

The entire disclosure of Japanese Patent Application No. 2009-249436,filed Oct. 29, 2009 is expressly incorporated by reference herein.

BACKGROUND

1. Technical Field

The present invention relates to a technology that measures the strengthof a magnetic field.

2. Related Art

As the technology that measures the strength of a magnetic field, anatom magnetization sensor that uses an electronic spin polarization ofalkali metal gas is suggested. When a pump light of a circular polarizedlight is irradiated with respect to a cell in which an alkali metalvapor is present and a probe light of the straight polarized lightpasses through the cell, the polarized light surface of the probe lightrotates according to the strength of the magnetic field that is appliedto the cell. Thus, the strength of the magnetic field is measured bydetecting a polarization rotation angle of the probe light.

A technology has been developed such that when the strength of amagnetic field is measured, a magnetic field gradient is measured usingtwo cells that are disposed at different positions to each other from anobject being measured, because magnetic fields that are caused byterrestrial magnetism, electric noise or the like in addition to themagnetic field of the object being measured may also get measured (forexample, JP-A-2009-162554).

According to the technology described in JP-A-2009-162554, even if themagnetic field that is generated from the object being measured has lowstrength, there is an advantage in that the magnetic field can beprecisely measured. However, the measuring precision may be low when thestrength of the magnetic field and the polarization rotation angle ofthe probe light are not in a proportionate relation. Given that anincrease in the strength of the magnetic field and the maintenance ofthe proportionate relation cannot be performed simultaneously, thestrength of all the magnetic fields in a measuring environment needs tobe restrained so as not to exceed any upper limit of the strength andthe measurable strength of the magnetic field of the object beingmeasured is greatly restricted. Therefore, there is a need to use anexpensive magnetic shield.

Also, the probe light is absorbed by the influence of the alkali metalvapor of the cell through which the probe light passes. There is a casewhere if the strength of the probe light is different when passingthrough the cell, the amount of change in the polarization rotationangle may be different even in a magnetic field of the same strength, sothat when the absorption amount becomes large, the relation of thepolarization rotation angle with respect to the magnetic field in thetwo cells causes them to differ from each other and the magnetic fieldgradient cannot be precisely measured.

SUMMARY

An advantage of some aspects of the invention is that a technology isprovided in which the precision of the measuring of the magnetic fieldgradient using the light pumping method can be improved.

According to an aspect of the invention there is provided a magneticfield measuring apparatus includes a probe light irradiating unit thatirradiates a probe light of a straight polarized light, a magneticmedium that is present in two regions through which the probe lightirradiated from the probe light irradiating unit passes through and ismagnetized according to a direction in which a pump light of a circularpolarized light is irradiated, the magnetic medium rotating a polarizedlight surface of the passing probe light by Faraday effect according toa strength of a component of a first direction orthogonal to a directionin which the probe light passes through the magnetic field that isapplied to each of the two regions from the outside, a light pathcontrol unit that controls the light path of the probe light so thateach of the probe lights passes through the magnetic medium in each ofthe two regions in the same number of times, a pump light irradiatingunit that irradiates a pump light of the circular polarized light withrespect to the magnetic medium so that the magnetic medium is magnetizedalong a direction other than the first direction in each of two regions,and a detection unit that detects a difference between a rotation amountof polarized light surface of the probe light according to which themagnetic medium passes through one of two regions a number of times anda rotation amount of polarized light surface of the probe light inaccordance with the magnetic medium passing through the other of tworegions a number of times.

According to an aspect of the invention, even if the polarizationrotation angle is small with respect to the strength of the magneticfield in each of the regions, the probe lights pass through a number oftimes with respect to the magnetic medium in each of the regions so thatthe polarization rotation angle that is detected is large and themeasuring precision of the magnetic field gradient can be improved.Also, even though the polarization rotation angle may be small in onethrough passage, the density of the magnetic medium becomes small sothat the absorption of the probe light can be small and the measuringprecision of the magnetic field gradient can be further improved.

It is preferable that the light path control unit has reflection mirrorsthat are provided so as to insert two regions and the probe light isreflected by the reflection mirrors so that the probe light passesthrough each of two regions a number of times with respect to themagnetic medium.

According to the magnetic field measuring apparatus, the probe light canpass through the two regions with respect to the magnetic medium anumber of times with brief configuration.

It is preferable that the light path control unit has corner cubes thatare provided so as to insert two regions and the probe light isreflected by the corner cubes so that the probe light passes througheach of two regions a number of times with respect to the magneticmedium.

According to the magnetic field measuring apparatus, the probe light canpass through the two regions with respect to the magnetic medium anumber of times with brief configuration.

It is preferable that the probe light irradiating unit has an ejectingsection that ejects the probe light, the detection unit has a lightreceiving section that receives the probe light and in which a polarizedlight surface is rotated in accordance with the magnetic medium passingthrough each of two regions a number of times, the ejecting section andthe light receiving section are positioned in the same side with respectto the magnetic medium.

According to the magnetic field measuring apparatus, the object beingmeasured can be positioned near one of the two regions that is away fromthe output section and light receiving section.

It is preferable that the magnetic field measuring apparatus furtherincluding a calculation unit which calculates a difference of strengthin the component of the first direction of the magnetic fields that areapplied to each of the two regions from the result of the detection bythe detection unit.

According to the magnetic field measuring apparatus, the strength of themagnetic field that is generated from the object being measured that ispositioned near one of the two regions can be measured.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a block diagram showing a configuration of a magnetic fieldmeasuring apparatus.

FIG. 2 is a drawing showing a configuration of a measuring section.

FIG. 3 is an illustrative view showing a magnetization change of amagnetic medium in a first gas cell and a second gas cell.

FIG. 4 is a drawing showing a relation between relative Larmor frequencyand y-axis direction magnetization.

FIG. 5 is a drawing showing a relation between the strength of amagnetic field that is applied and a polarized light rotation angle.

FIGS. 6A to 6H are illustrative views showing rotation of a polarizedlight surface by the passing of a probe light through gas cells.

FIG. 7 is a drawing showing a configuration of a measuring sectionaccording to a first modified example.

FIG. 8 is a drawing showing a configuration of a measuring sectionaccording to a second modified example.

FIG. 9 is a drawing showing a configuration of a measuring sectionaccording to a fourth modified example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Embodiments Overall Configuration

FIG. 1 is a block diagram showing a configuration of a magnetic fieldmeasuring apparatus 1. The magnetic field measuring apparatus 1 is anapparatus that measures the strength of a magnetic field that isgenerated from an object being measured using a light pumping method,and has the measuring section 10, the control section 11, the storingsection 12, the display section 13, the operation section 14 and theinterface 15. The measuring section 10 has each of the configurationsthat measures the strength of the magnetic field and performs control ofthe measuring start and finish by the control section 11. The measuringsection 10 outputs polarized light rotation angle information to thecontrol section 11 during measuring. Description will be given belowregarding the detail of the measuring section 10 and the polarized lightrotation angle information.

The control section 11 has an arithmetic processing circuit such as CPU(Central Processing Unit) or the like, ROM (Read Only Memory), RAM(Random Access Memory), and the like, and the CPU loads a program thatis stored in the ROM to the RAM and performs the program. Thus, thecontrol section 11 performs control of each section of the magneticfield measuring apparatus 1 through a bus. Also, the control section 11also has a function as a calculation unit that performs the calculationprocessing that calculates the measured strength of a magnetic fieldfrom the polarized light rotation information as described below.

The storing section 12 is a storing unit of a hard disc and nonvolatilememory, or the like, and stores all kinds of programs, the magneticfield measuring result and the like.

The display section 13 is a display device that has a display screensuch as liquid crystal display or the like, and display contents arecontrolled by the control section 11. The display contents are all kindsof setting screen, display of the measuring results and the like, forexample.

The operation section 14 has operation members such as buttons,keyboard, touch panel or the like, and data that show the operationcontents are output to the control section 11 when a user operates theoperation members.

The interface 15 uses a terminal such as USB (Universal Serial Bus) orthe like and uses a communication function that is connected to acommunication network so that it has a connecting function forexchanging information with an external apparatus.

Next, description will be given regarding the configuration of themeasuring section 10.

Configuration of Measuring Section 10

FIG. 2 is a drawing showing a configuration of a measuring section 10.The measuring section 10 has the first gas cell 111, the second gas cell112, the reflecting mirrors 121 and 122, the pump light irradiatingsection 130, the probe light irradiating section 140 and the probe lightdetecting section 150. As shown in FIG. 2, description will be givenwherein the right direction of the drawing is the x-axis direction, theupper direction of the drawing orthogonal with respect to the x-axis isthe y-axis direction, and the vertical front direction of the papersurface orthogonal with respect to a plane of the x-axis and the y-axisis the z-axis direction. In the embodiment, the measuring section 10 isdescribed as a configuration for measuring the strength of a magneticfield in the z-axis direction. As shown in FIG. 2, the object beingmeasured object 1000 is positioned nearer the first gas cell 111 thanthe second gas cell 112 when measuring the strength of the magneticfield that is formed by the object being measured object 1000.

The first gas cell 111 and the second gas cell 112 are arranged parallelin the y-axis direction. The first gas cell 111 and the second gas cell112 (hereinafter, simply referred to as gas cells when it is notnecessary to distinguish each of them individually) are hollow membersthat are constituted from a material such as glass, plastic or the likethrough which a pump light and a probe light are passed as describedbelow. In the hollow portion of the gas cell, a magnetic medium of analkali metal atom such as potassium (K), rubidium (Rb), cesium (Cs) orthe like is sealed. Also, rare gases such as helium (He), argon (Ar) orthe like and nonmagnetic gas such as nitrogen (N) may be sealed as abuffer gas. When measuring the strength of the magnetic field, theinterior of the gas cell is heated by a heating unit (not shown) andvapor of the alkali metal atom fills the interior of the gas cell and isgasified. The heating is not needed in the case of an atom density inits gasified state that is sufficient without being heated.

As the magnetic medium that is sealed within the gas cell, the alkalimetal atom is used, however, as described below, the magnetic medium isnot limited to the alkali metal atom, if it is magnetized according toan irradiating direction when a pump light of circular polarized lightis irradiated.

As described above, the first gas cell 111 and the second gas cell 112are maintained in two regions that are magnetic field measuring regionsso that a magnetic medium is present. Thus, two gas cells are notnecessarily needed, and it may be maintained in a single gas cell. Thetwo regions that become the magnetic field measuring regions are regionsin which the pump light 131 (described below) irradiates the magneticmedium within the first gas cell 111 and a region in which the pumplight 132 (described below) irradiates the magnetic medium within thesecond gas cell 112. The regions are regions in which the pump lights131 and 132 are irradiated. However, the regions may also includeregions in which the magnetic medium that is magnetized by the pumplight are continuously influenced by the magnetization, and diffused andwidened.

In the description below, the first gas cell 111 and the second gas cell112 correspond two regions that are magnetic field measuring regions.Also, the first gas cell 111 and the second gas cell 112 have the samedimensions at least in the y-axis direction, in other words, thedimensions of the two regions in the y-axis direction are the same aseach other. Moreover, the first gas cell 111 and the second gas cell 112also have the same atom density of the magnetic medium as each other.Also, the first gas cell 111 and the second gas cell 112 are not limitedto the above-described configurations, if a relation between thestrength of the magnetic field Bz and a polarized light rotation angle θof the polarized light surface of the probe light 141 are the same aseach other respectively as described below.

The reflecting mirrors 121 and 122 are provided so as to insert thefirst gas cell 111 and the second gas cell 112, and are light pathcontrol units that pass through each of probe lights 141 the same numberof times (in the embodiment, four times respectively) with respect tothe first gas cell 111 and the second gas cell 112 so as to reflect theprobe light 141 and control the light path. The reflecting mirror 121 isprovided at the first gas cell 111 side and the reflecting mirror 122 isprovided at the second gas cell 112 side. Also, the reflecting mirror122 has a configuration (that is, a introducing hole in the embodiment)in which the probe light 141 that is ejected from an ejecting section1401 of the probe light irradiating section 140 which is outside betweenthe reflecting mirrors 121 and 122 is guided to the inside, and theprobe light 141 that is reflected by the reflecting mirrors 121 and 122,and is within the mirrors is guided to the light receiving section 1501of the probe light detecting section 150 of the outside.

The pump light irradiating section 130 is a pump light irradiating unitthat has the irradiating sections 1301 and 1302 irradiating the pumplights 131 and 132 that are pumping light in the light pumping method tothe first gas cell 111 and the second gas cell 112 respectively. Thepump light irradiating section 130 irradiates the pump lights 131 and132 in the same direction respectively (irradiating in the x-axispositive direction as shown in FIG. 2). Also, the pump lights 131 and132 are circular polarized lights respectively that are irradiated bythe pump light irradiating section 130 and are circular polarized lightsthat rotate in the counter direction respectively. In the embodiment,the pump light 131 is a left circular polarized light (σ+: the circularpolarized light that rotates clockwise (left rotation seen from thelight receiving side) with respect to the light advancing direction (thex-axis positive direction)) and the pump light 132 is right circularpolarized light (σ−: the circular polarized light that rotatescounterclockwise (right rotation seen from the light receiving side)with respect to the advancing direction of the light (the x-axispositive direction)).

The pump light irradiating section 130 has one light source and convertsthe light that is generated from the light source to the pump lights 131and 132 using an optical system such as a phase difference plate, a beamsplitter, a mirror or the like, and then irradiates the light. Also, thepump light irradiating section 130 may have two light sources and lightthat is generated from each of the light sources may be irradiated asthe pump lights 131 and 132.

The probe light irradiating section 140 is a probe light irradiatingunit having the ejecting section 1401 that ejects the probe light 141 ofstraight polarized light. As described above, the probe light 141 isreflected in the reflecting mirrors 121 and 122, passes a number oftimes through the first gas cell 111 and the second gas cell 112respectively, and reaches the light receiving section 1501 of the probelight detecting section 150. In the embodiment, the ejecting section1401 and the light receiving section 1501 are provided in the same sidewith respect to the first gas cell 111 and the second gas cell 112, andin the embodiment, are provided on the side opposite the second gas cell112 through the reflecting mirror 122. According to the abovedescription, the object being measured 1000 can be positioned in thevicinity of the first gas cell 111 and the magnetic field from theobject being measured 1000 can be effectively measured.

The probe light irradiating section 140 has a light source and the lightthat is generated from the light source may pass through the opticalsystem such as a lens or the like until the probe light 141 is ejectedfrom the ejecting section 1401.

As illustrated in FIG. 2, a light path of the probe light 141 is notcompletely parallel but substantially parallel with the y-axis and theparallel is assumed in the description below.

The probe light detecting section 150 has the light receiving section1501 that receives the probe light 141 and is a detecting unit thatdetects the polarized light rotation angle of the received probe light141. For example, the probe light detecting section 150 may use apolarized light beam splitter and a photo detector, or may use apolarimeter.

The probe light detecting section 150 sets in advance a state of thepolarized light surface of the probe light 141 that is irradiated fromthe probe light irradiating section 140, measures that how many timesthe polarized light surface of the probe light 141 that is received atthe light receiving section 1501 is rotated in which the polarized lightsurface is reference, and detects the rotation amount as the polarizedlight rotation angle.

The polarized light rotation angle that is detected as described aboveis indicated as a difference between the rotation amount of thepolarized light surface of the probe light 141 by the first gas cell 111and the rotation amount of the polarized light surface of the probelight 141 by the second gas cell 112, as a result of the probe light 141passing a number of times through the first gas cell 111 and the secondgas cell 112 as described below.

The probe light detecting section 150 detects the polarized lightrotation angle information indicating the detected polarized lightrotation angle to the control section 11. Next, description will begiven regarding the magnetization change in the magnetic medium that issealed in the first gas cell 111 and the second gas cell 112.

FIG. 3 is an illustrative view showing a magnetization change in amagnetic medium in the first gas cell 111 and a second gas cell 112. Themagnetic medium such as alkali metal or the like in the gas cell ismagnetized in which when the light of the circular polarized light isirradiated, an electron spin polarization of the alkali metal atom isgenerated in accordance with the light advancing direction. In a casewhere the irradiated light is left circular polarized light (σ+),magnetization is along the light advancing direction, while in a casewhere the irradiated light is right circular polarized light (σ−),magnetization is in the reverse of the light advancing direction. Such aphenomenon is referred to as light pumping. Thus, when the lightirradiation is stopped, the spin polarization is diminished and isdirected in a random direction so that a magnetization vector becomessmall with the elapse of time. The speed at which the spin polarizationis generated and magnetization is decreased to a specific ratio (forexample, 1/e) is a lateral diminishing speed Γg.

Here, as shown in FIG. 3, the pump light 131 of left circular polarizedlight (σ+) is irradiated in the x-axis positive direction at the firstgas cell 111 so that the magnetic medium is magnetized in the x-axispositive direction at the first gas cell 111 (magnetization vector M1).Meanwhile, the pump light 132 of right circular polarized light (σ−) isirradiated in the x-axis negative direction at the second gas cell 112so that the magnetic medium is magnetized in the x-axis negativedirection at the second gas cell 112 (magnetization vector M2).

Also, when the magnetic field is applied to the magnetic medium at thefirst gas cell 111 and the second gas cell 112, the electron spin startsa Larmor processional motion and rotates within the surface that isvertical to the applied magnetic field direction. The Larmor frequencyω_(L) is proportionate to the size of the applied magnetic field andω_(L)=γB (γ: magnetic rotation ratio, B: strength of applied magneticfield).

As shown in FIG. 3, a magnetic field B1 is applied to the first gas cell111 in the z-axis positive direction and a magnetic field B2 is appliedto the second gas cell 112 in z-axis positive direction. The directionin which the magnetic field is applied is in practice not limited to thez-axis positive direction and description is given focusing on thez-axis positive direction component of the strength of the magneticfield.

When the magnetic field is applied in the z-axis positive direction, themagnetization vector is rotated (rotated counterclockwise within the xyplane, the xy plane as seen from the z-axis positive direction) by theLarmor processional motion. Thus, the magnetization vector is rotatedand the size thereof becomes small with the elapse of time.

Next, as the result, description will be given regarding themagnetization size generated in the y-axis direction.

FIG. 4 is a drawing showing a relation between a relative Larmorfrequency and the y-axis direction magnetization My. The relative Larmorfrequency is present as Larmor frequency ω_(L)/lateral diminishing speedΓg. The magnetization My of the y-axis direction is represented asMy=−Cω_(L)Γg/(ω_(L) ²Γg²), and shows the size of the magnetizationvector of the y-axis direction that is generated by the rotation thereofon the basis of the magnetization vector (corresponding to themagnetization vector M2) toward the x-axis negative direction. C is aconstant that is determined according to the strength of the probe light141 and the density of the magnetic medium within the gas cell. Also,ω_(L) makes the counterclockwise rotation seen from z-axis positivedirection within xy plane as a positive value.

In the state in which the magnetic medium within the gas cell ismagnetized, the polarized light surface of the probe light is rotated bythe Faraday effect when the probe light 141 passes through the magneticmedium. Specifically, the polarized light surface is rotated clockwisewith respect to the advancing direction according to the size ofmagnetization along the direction in which the probe light 141 isadvanced. In other words, when the probe light 141 is advanced in they-axis positive direction, the polarized light surface of the probelight 141 is rotated according to the polarization size in the y-axisdirection of magnetic medium within the passing gas cells. As describedabove, the magnetization size in the magnetic medium within the gascells is changed according to the strength of the magnetic field that isapplied as shown in FIG. 4. Accordingly, the polarized light surface ofthe probe light 141 is rotated according to the strength of the magneticfield that is applied to magnetic medium within the gas cells from theoutside.

Next, description will be given regarding the relation between thestrength of the magnetic field that is applied from the outside and therotation amount (the polarized light rotation angle) of the polarizedlight surface of the probe light 141.

FIG. 5 is a drawing showing the relation between the strength of amagnetic field Bz that is applied and a polarized light rotation angleθ. The magnetic field Bz shows the strength of the magnetic field thatis applied in the z-axis positive direction. The polarized lightrotation angle θ defines the counterclockwise direction with respect tothe advancing direction of the probe light 141 as positive. In otherwords, in a case where the advancing direction of the probe light 141and the magnetization vector of the y-axis direction are the samedirection, the polarized light surface of the probe light 141 is rotatedclockwise with respect to the advancing direction according to the sizeof the magnetization vector, while in the case of the oppositedirection, it is rotated counterclockwise.

Also, a solid line P and a dot line Q are assumed such that the atomdensity of the magnetic mediums are different to each other and the atomdensity of the dot line Q is lower than that of the solid line P.

The solid line P is described with reference to FIG. 5. When an absolutevalue of the strength of the magnetic field Bz becomes large, thepolarized light rotation angle θ proportionally becomes large, and theproportionate relation is maintained only when the strength of themagnetic field Bz is within a magnetic field measuring range Br0. Thus,regarding the strength of the magnetic field Bz outside the magneticfield measuring range Br0, measurement cannot be performed exactly. Themagnetic field measuring range Br0 is determined by Br0=2Γg/γ, since theabsolute value of the Larmor frequency ω_(L)/Γg shown in FIG. 4 issubstantially less than 1.

Meanwhile, regarding the dot line Q, the atom density of the magneticmedium is low and thus Γg becomes large so that the change of thepolarized light rotation angle θ with respect to the change in thestrength of the magnetic field Bz becomes small. In other words, theatom density becomes low and then sensitivity with respect to themagnetic field Bz becomes low so that the magnetic field measuring rangecan be widened from Br0 to Br1.

Next, description will be given regarding the rotation of the polarizedlight surface of the probe light 141.

Rotation of Polarized Light Surface of Probe Light 141

FIGS. 6A to 6H are illustrative views showing the rotation of thepolarized light surfaces by passing through the gas cells of the probelight 141. The magnetic field B0 is applied in the z-axis positivedirection to the second gas cell 112 and the magnetic field B0+Bh isapplied in the z-axis positive direction to the first gas cell 111. Thedifference between the magnetic fields occurs through the differencebetween the distances from the object being measured 1000 to the firstgas cell 111 and to the second gas cell 112. The magnetic fields B0 andB0+Bh are within the above-described magnetic field measuring range.

A magnetization vector M2 y of the y-axis direction in the magneticmedium of the second gas cell 112 is directed toward the y-axis negativedirection and a magnetization vector M1 y of the y-axis direction in themagnetic medium of the first gas cell 111 is directed toward the y-axispositive direction by the magnetic fields B0 and B0+Bh. Since themagnetic field with regard to the first gas cell 111 is larger than themagnetic field with regard to the second gas cell 112, the size of themagnetization vector M1 y is larger than that of the magnetizationvector M2 y. In other words, the absolute value (hereinafter, referredto as the first polarized light rotation angle α) of the polarized lightrotation angle θ of the probe light 141 in the first gas cell 111 islarger than the absolute value (hereinafter, referred to as the secondpolarized light rotation angle β) of the polarized light rotation angleθ of the probe light 141 in the second gas cell 112.

In the embodiment, description is given in the case where the probelight 141 advances to the y-axis negative direction (referred to asprobe light 141-1), passes through the second gas cell 112 and the firstgas cell 111, and then is reflected by the reflecting mirror 121,advances to the y-axis positive direction (referred to as probe light141-2), passes through the first gas cell 111 and the second gas cell112. Moreover, FIGS. 6A to 6H show the direction of the polarized lightsurface with respect to the advancing direction of the probe light 141.In FIGS. 6A to 6H, the clockwise rotation with respect to the advancingdirection of the probe light 141 is defined as positive.

First of all, before the probe light 141-1 passes through the second gascell 112, as shown in FIG. 6A, the polarized light surface of the probelight 141-1 is along the z-axis. When the probe light 141-1 passesthrough the second gas cell 112, as shown in FIG. 6B, the polarizedlight surface rotates by +β (β in clockwise rotation with respect to theadvancing direction of the probe light 141-1) since the advancingdirection of the probe light 141-1 and the direction of themagnetization vector M2 y are in the same direction as each other. Whenthe probe light 141-1 further passes through the first gas cell 111, asshown in FIG. 6C, the polarized light surface rotates by −α (α incounterclockwise rotation with respect to the advancing direction of theprobe light 141-1) because the advancing direction of the probe light141-1 and the direction of magnetization vector M1 y are in the oppositedirection to each other.

Accordingly, as shown in FIG. 6D, the polarized light surface of theprobe light 141-1 that passed through the second gas cell 112 and thefirst gas cell 111 rotates by β-α compared to before the passing throughis performed. The polarized light rotation angle (β-α) corresponds tothat of the magnetic field Bh.

Next, when the probe light 141-1 is reflected by the reflecting mirror121, the advancing direction is reversed to the y-axis positivedirection (the probe light 141-2). Thus, the phase of probe lightrotates 180° and as shown in FIG. 6E, the polarized light surface withrespect to the advancing direction rotates by the polarized lightrotation angle (α-β) with respect to the original polarized lightsurface shown in FIG. 6A.

When the probe light 141-2 passes through the first gas cell 111, asshown in FIG. 6F, the polarized light surface rotates by +α (α inclockwise rotation with respect to the advancing direction of the probelight 141-2) because the advancing direction of the probe light 141-2and the direction of the magnetization vector M1 y are in the samedirection as each other. Also, when the probe light 141-2 further passesthrough the second gas cell 112, as shown in FIG. 6G, the polarizedlight surface rotates by −β (β in counterclockwise rotation with respectto the advancing direction of the probe light 141-2) because theadvancing direction of the probe light 141-2 and the direction ofmagnetization vector M2 y are in the opposite direction to each other.

Accordingly, as shown in FIG. 6H, the probe light 141 that passedthrough the second gas cell 112 and the first gas cell 111, reflected bythe reflecting mirror 121 and then passed again through the first gascell 111 and the second gas cell 112, rotates the polarized lightsurface by 2α-2β compared to before passing through is performed. Thepolarized light rotation angle 2α-2β corresponds to twice the polarizedlight rotation angle α-β with respect to the magnetic field Bh.

In a case where the probe light 141 is irradiated as shown in FIG. 2,the rotation of the above-described polarized light surface is performedonce more, so that the polarized light rotation angle becomes 4α-4β andthen becomes four times the polarized light rotation angle α-β withrespect to the magnetic field Bh.

The polarized light rotation angle α-β with respect to the magneticfield Bh becomes a small rotation angle as the atom density of themagnetic medium in the gas cell is low and the magnetic field measuringrange is widened. For example, when the atom density is 1/16(=(¼)²), themagnetic field measuring range becomes four times, in other words, thepolarized light rotation angle becomes ¼ even in a magnetic field of thesame strength. In the configuration of the embodiment, the number ofthrough passes wherein the probe light 141 is reflected by thereflecting mirrors 121 and 122 and passed through the first gas cell 111and the second gas cell 112 is a number of times (four times), so thatthe polarized light rotation angle can be obtained four times even in amagnetic field of the same strength.

Also, the number of through passes wherein the probe light 141 passedthrough the first gas cell 111 and the second gas cell 112 a number oftimes, so that the strength of the probe light 141 is also decreased,however in the embodiment, the decreasing of the strength of the probelight 141 can be small, since the atom density of the magnetic medium islow in the first gas cell 111 and the second gas cell 112 to widen themagnetic field measuring range.

In the embodiment, the probe light 141 has the configuration in whichthe probe light 141 passes through the first gas cell 111 and the secondgas cell 112 as a set that is related to the measuring of the magneticfield gradient, and then reflected by the reflecting mirrors 121 and122. Thus, the difference between the strength of the probe light 141when the probe light 141 passes through the first gas cell 111 or thesecond gas cell 112 and the strength of the probe light 141 when theprobe light 141 passes through the next gas cell is small, so thatinfluence thereof on the measuring is small. If the decreasing amount isknown beforehand, the decreasing amount of the polarized light rotationangle can be amended since the polarized light rotation angle isdecreased according to the decreasing amount. Accordingly, even in thecase where the decreasing amount of the strength of the probe light 141accumulates and becomes large while the probe light 141 is ejected fromthe ejecting section 1401 and received in the light receiving section1501, the influence thereof on the measuring is small.

As the result of the detection, the control section 11 obtains thepolarized light rotation angle information that is calculated from themeasuring section 10, and calculates the strength of magnetic field Bhthat is the difference between the magnetic field B0+Bh that is appliedto the magnetic medium in the first gas cell 111 and the magnetic fieldB0 that is applied to the magnetic medium in the second gas cell 112.According to the above-described strength, the strength of the magneticfield that is generated by the object being measured 1000 can bemeasured.

Specifically, the relation between the magnetic field Bz and thepolarized light rotation angle θ corresponding to FIG. 5 is stored inthe control section 11 and the polarized light rotation angle that isindicated in the polarized light rotation angle information is ¼ so thatthe corresponding strength of magnetic field may be calculated. Asdescribed above, in a case where the amendment is performed taking intoconsideration the strength of the probe light 141, the polarized lightrotation angle that is indicated in the polarized light rotation angleinformation is not ¼, but may be 1/(4−c), using amendment value c forexample.

As described above, the magnetic field measuring apparatus 1 accordingto the embodiment of the invention reflects the probe light 141 usingthe reflecting mirrors 121 and 122 at the measuring section 10 andpasses through the first gas cell 111 and the second gas cell 112 anumber of times so that the polarized light rotation angle in the probelight 141 can be large. At this time, the atom density of the magneticmedium in the first gas cell 111 and the second gas cell 112 becomessmall so that the decreasing amount of the probe light 141 can besuppressed and the magnetic field measuring range can be increased.

Modified Embodiment

As described above, description has been given concerning the embodimentof the invention, however the invention can be modified in variousexamples as described below.

First Modified Example

In the above-described embodiment, the ejecting section 1401 and thelight receiving section 1501 are positioned in the same side withrespect to the first gas cell 111 and the second gas cell 112, howeverthey may be positioned in different sides to each other.

FIG. 7 is a drawing showing a configuration of a measuring section 10Aaccording to a first modified example. The measuring section 10A usesthe reflecting mirrors 121A and 122A instead of the reflecting mirrors121 and 122 of the measuring section 10 in the embodiment, and theposition of the light receiving section 1501 is on the side opposite tothe position of the ejecting section 1401 through the first gas cell 111and the second gas cell 112.

In the case of this configuration, the relation between positive andnegative of the y-axis direction is different when the light receivingsection 1501 receives the probe light 141, so that the process isperformed reversing positive and negative as the above-describedembodiment when the polarized light rotation angle information isprocessed in the control section 11.

Second Modified Example

In the above-described embodiment, the reflecting mirrors 121 and 122are used as the light path control units, however a unit other than thereflection mirrors may control the light path of the probe light 141.

FIG. 8 is a drawing showing a configuration of a measuring section 10Baccording to a second modified example. The measuring section 10B usesright angle prisms 123 and 124 instead the reflecting mirrors 121 and122 of the measuring section 10 of the embodiment. Even in thisconfiguration, the probe light 141 can pass through the first gas cell111 and the second gas cell 112 a number of times.

When the advancing direction of the probe light 141 is changed in 180°by the right angle prisms 123 and 124, the right angle prisms 123 and124 may be constituted so as to rotate the phase of the probe light 141180° similarly to the reflecting mirrors 121 and 122 of the embodiment.Also, the right angle prisms 123 and 124 themselves do not constitutethe example but other optical systems may separately be provided in themeasuring section 10B to realize such configuration.

Also, the reflection surfaces of the right angle prisms 123 and 124 aretwo, however the reflection surface of either of the right angle prisms123 and 124 may be three of corner cube.

Third Modified Example

In the above-described embodiment, the pump lights 131 and 132 irradiatein the same direction as each other and the direction of the circularpolarized lights are different to each other, however the directions ofthe circular polarized lights may be in the same direction as each otherand the irradiation directions may be in the opposite direction to eachother. For example, the pump lights 132 may be left circular polarizedlight the same as the pump light 131 and the irradiation direction maybe the x-axis negative direction.

Also, the invention is not limited to the example, and when the pumplights 131 and 132 irradiate to the first gas cell 111 and the secondgas cell 112 respectively, the invention may have a configuration suchthat each of the magnetization directions of the magnetic mediums may beopposite to each other.

Accordingly, the pump light irradiating section 130 may have aconfiguration in which as the result of the probe light 141 passingthrough the first gas cell 111 and the second gas cell 112 a number oftimes, the polarized light rotation angle that is detected at the probelight detecting section 150 becomes the difference between the rotationamount of the polarized light surface of the probe light 141 through thefirst gas cell 111 and the rotation amount of the polarized lightsurface of the probe light 141 through the second gas cell 112.

Fourth Modified Example

In the embodiment as described above, when the pump lights 131 and 132irradiate the first gas cell 111 and the second gas cell 112respectively, the example has a configuration such that themagnetization directions of the magnetic mediums are opposite to eachother, however the pump lights 131 and 132 may be left circularpolarized light together so that the magnetization directions may be thesame as each other and a half-wavelength plate may be used.

FIG. 9 is a drawing showing a configuration of a measuring section 10Caccording to a fourth modified example. The measuring section 10C mayuse a half-wavelength plate 160 at a region on which the probe light 141between the first gas cell 111 and the second gas cell 112 passesthrough. Thus, when the probe light 141 passes the half-wavelength plate160, the phases are deviated 180°, so that the rotation directions ofthe polarized light surface of the probe light 141 in the first gas cell111 and the second gas cell 112 can be opposite to each other and theeffect can be the same as the embodiment.

Accordingly, a configuration in which the rotation of the polarizedlight surface of the probe light 141 is controlled may be providedtherein as the result that the probe light 141 passes the first gas cell111 and the second gas cell 112 a number of times, the polarized lightrotation angle that is detected at the probe light detecting section 150becomes the difference between the rotation amount of the polarizedlight surface of the probe light 141 by the first gas cell 111 and therotation amount of the polarized light surface of the probe light 141 bythe second gas cell 112.

What is claimed is:
 1. A magnetic field measuring apparatus comprising:a probe light irradiating unit that irradiates a probe light beam in afirst direction, the probe light beam having a straight polarization; apump light irradiating unit that irradiates first and second pump lightbeams in a second direction that is different from the first direction,the first and second pump light beams having first and second circularpolarizations, respectively; first and second magnetic mediums throughwhich the probe light beam passes, the first and second magnetic mediumsbeing magnetized with respect to a polarization direction of the firstand second circular polarizations, a light path control unit thatcontrols a light path of the probe light beam so that the number ofpassage times of the probe light beam through the first and secondmagnetic mediums are the same, and a detection unit that receives theprobe light beam and that detects a difference between first and secondrotation amounts of first and second polarized light surfaces of theprobe light beam, wherein the first magnetic medium rotates the firstpolarized light surface of the probe light beam according to firststrength of a first component in a perpendicular direction perpendicularto the first direction of an externally applied magnetic field that isapplied to the first magnetic medium by Faraday effect, the secondmagnetic medium rotates the second polarized light surface of the probelight beam according to second strength of the first component in theperpendicular direction of the externally applied magnetic field that isapplied to the second magnetic medium by Faraday effect, the probe lightirradiating unit, the detection unit, and the first and second magneticmediums are located along the first direction, and the probe lightirradiating unit and the detection unit are located at the same sidenext to one of the first and second magnetic mediums, the light pathcontrol unit includes first and second reflection mirrors between whichthe first and second magnetic mediums are located, the reflectionmirrors reflect the probe light beam so that the probe light beam passesthrough the first and second magnetic mediums the same number of times,the first reflection mirror is located directly adjacent to the probelight irradiating unit, the detection unit and the one of the first andsecond magnetic mediums, and the second reflection mirror is locateddirectly adjacent to the other of the first and second magnetic mediums,and the detection unit detects the difference between the first andsecond rotation amounts of the first and second polarized light surfacesof the probe light beam in consideration of a decreasing amount ofstrength of the probe light beam.
 2. The magnetic field measuringapparatus according to claim 1, wherein the light path control unit hascorner cubes, and the corner cubes reflect the probe light beam so thatthe probe light beam passes through the first and second magneticmediums the same number of times.
 3. The magnetic field measuringapparatus according to claim 1, further including: a calculation unitthat calculates a difference between the first strength and the secondstrength.